Formation and Reactions of Negative Ions Relevantto Chemical Ionization Mass Spectrometry. I. Cl

16 Pages·2006·2.31 MB·English

Environmental HealthPerspectives
Vol. 36, pp. 47-62, 1980
Formation and Reactions of Negative
Ions Relevant to Chemical Ionization
Mass Spectrometry. I. Cl Mass
Spectra of Organic Compounds
Produced by F- Reactions
by T. 0. Tiernan*, C. Chang*, and C. C. Cheng*
Asystematic studyofthenegative-ionchemicalionizationmassspectraproducedbythere-
actionofF-withawidevarietyoforganiccompoundshasbeenaccomplished.Atime-of-flight
mass spectrometerfittedwith amodified high pressure ion sourcewas employedforthese ex-
periments. The F- reagent ion was generated from CF3H or NF3, typically at an ion source
pressure of100,um.InpureNF3,F- isthe majorionformedandconstitutes morethan90% of
thetotal ionintensity. WhileF-isalsothemajorprimary ionformedinpureCF3H,itunder-
goes rapid ion-molecule reactions at elevated source pressures, yielding (HF),F- (n - 1-3)
ions,whichmakesCF3Hlesssuitableasachemicalionization reagentgas.Amongtheorganic
compounds investigated were carboxylic acids, ketones, aldehydes, esters, alcohols, phenols,
halides, nitriles, nitrobenzene, ethers,aminesandhydrocarbons. An intense (M - 1)- ion was
observedintheF-chemicalionizationmassspectraofcarboxylicacids,ketones,aldehydes and
phenols.Alcoholsyieldonly (M+F)-ionsuponreactionwithF-.Aweaker(M+F)-ion was
alsodetectedintheF-chemical ionization spectraofcarboxylicacids,aldehydes,ketones and
nitriles. The F- chemical ionization mass spectra ofesters, halides,nitriles, nitrobenzene and
ethersarecharacterizedprimarilybytheions, RCOO-,X-,CN-,NO2-,andOR-, respectively.
Inaddition,estersshowaveryweak(M-1)-ion(exceptformates).IntheF-chemical ioniza-
tion spectra ofsome aliphatic alkanesand o-xylene, avery weak (M + F)- ion was observed.
Aminesandaliphaticalkenesexhibitonlyinsignificantfragmentionsundersimilarconditions,
while aromatic hydrocarbons, such as benzene andtoluenearenotreactive atall withthe F-
ion.Themechanismsofthevariousreactionsmentionedarediscussed,andseveralexperimen-
tal complications are noted. In still other studies, the effects ofvarying several experimental
parameters, including source pressure, relative proportions of the reagent and analyte, and
otherionsourceparameters,ontheobservedchemicalionizationmassspectrawerealsoinves-
tigated.InamixtureofNF3and n-butanol,forexample,theratiooftheintensities ofthe ions
characteristic ofthealcoholtothatofthe (HF),,F- ionwas foundtodecreasewith increasing
samplepressure,withincreasingNF3pressure,andwithincreasingelectronenergy.Nosignifi-
canteffectsonthespectrawereobservedtoresultfromvariationofthesourcerepellerfieldor
the source temperature. The addition ofargontothe source as apotential moderatordid not
altertheF-chemicalionizationspectrumsignificantly,buttheuseofoxygenappearstoinhibit
formationofthe (HF),F-clusterion.TheadvantagesofusingF-asachemicalionization re-
agent arediscussed,andcomparisons aremadewithotherreagent ions.
Introduction ten years, as clearly indicated by the attendance
at the present and other recent symposia (1) con-
a e n d r ns i c
Interest in the development and applications of
negative-ion chemical ionization (NICI) mass *BrehmLaboratoryandDepartmentofChemistry, Wright
spectrometry has expanded rapidly in the past State University, Dayton, Ohio45435.
November 1980 47
cernedwith thistopic. The numberofpublications formed. Thus the F- NICI mass spectra should be
appearing in the chemical literature which deal simpler and more readily interpreted than those
with NICI mass spectrometry has also increased resulting from reactions of other halide negative
significantly during this time period. Several cur- ions.
rent comprehensive reviews (2-4) have surveyed The experimental procedures employed and the
these publications. In spite of the increasing ac- results obtained in this investigation are de-
tivity in this field of research, however, relatively scribed in the following sections.
few negative ions have yet been investigated as
chemical ionization reagents. This is illustrated
by the listing shown in Table 1. This situation is Experimental
due to the lack of established methods for gener-
atingusable quantities of manytypes of negative The instrument utilized forthe experiments re-
ions, andtothe uncertainties which currently pre- ported herein is a Bendix Model 12-101 time-of-
vail in the understanding of gas phase negative flight mass spectrometer equipped with a 100 cm.
ion chemistry. drift tube. The mass spectrometer is fitted with a
In the present paper, we report the results of a specially designed, high pressure ion source,
systematic study of the reactions of the fluoride which was developed by Changand Tiernan (13).
negative ion, F-, with a variety of organic mole- Figure 1 shows a schematic diagram of the ion
cules, which are representative of different func- source and the focusing lens system. The source,
tional group classes. The F- ion was selected for which was constructed from a solid copper block,
investigation as a NICI reagent ion for several is actually an open cylinderwith an inside diame-
reasons. First, it was expected that F- could be terof 1.91 cm and alength of 1 cm. A gas inlet or-
readily produced in large abundance by electron ifice, 0.64 cm in diameter, was cut into the side of
impact on molecules such as CH3F and NF3. The the block. An electron entrance plate (EEP) and
appearance potential of F- from NF3 is very near an ion exitplate (IEP) are utilized to seal the top
0 eV (11), and previous experiments in our lab- and the bottom of the source cylinder. Both plates
oratory have demonstrated that the rate coef- were identically manufactured from stainless
ficient for attachment of near-thermal electrons steel, and are cone-shaped, with a 0.05 cm diame-
to NF3, yielding F-, was very large (12). Also, F- ter orifice at the cone apex. A -6V dc potential is
isthe onlyionicproductobservedfromlowenergy applied continuously to the electron entrance
electron attachment to NF3. Secondly, the proton plate as a repeller potential. This facilitates re-
affinity of F- is relatively high, 1548 KJ/mole, moval of ions from the source. The ion exit plate
which isjust between theproton affinities of OH-, is maintained at groundpotential. Bothplates can
(1632 KJ/mole) and Cl- (1393 KJ/mole) (2). The be used to monitor the electron beam current if
anionic nucleophilicity of F- also lies between desired, by suitable connections. A coiled tung-
that of OH- and Cl-, which have previously been sten wire filament is located about 1 cm from the
utilized in NICI studies (see Table 1). These con- electron entrance plate. In order to collimate the
siderations suggest that F- should be reactive electron beam, a beam-focusing cylinder, which
with a number of organic molecules. Still another was constructed from 90% transparent rhenium
feature of F- as a NICI reagent ion is that fluor- mesh screen, isused tosurroundthe filament. Use
ine is monoisotopic, sothat if F- associates with a of the transparent wire mesh permits efficient
particular molecule, only one (M + 19)- product pumping of neutral gas from the filament region.
ion will be observed, since noisotope peaks can be This focusing cylinder and one end of the fila-
Table 1. Negativeionsusedaschenlicalionizationmassspectrometryreagentsinstudiesreportedtodate.
Typeofion Typicaltotal
Reagention Sourcegases sourceused sourcepressure,torr Reference
H- H2 Electronimpact 1 (2)
O- N20/N2 Electronimpact 0.3-0.5 (5)
02 N20/N2/02 Electronimpact 0.3-0.5 (2)
02 02,H2/02 Townsenddischarge 1 (6,7)
02- Air Atmospherepressure Atmosphericpressure (8)
ionization
Cl- CH2Cl2 Electronimpact 1 (9)
OH- N20/H2,N20/CH4 Electronimpact 1-3 (10)
CH30- CH30NO/CH4 Electronimpact 1 (7)
48 Environmental HealthPerspectives
Gas inlet GI G2
i1Iil (-175v) (GD)
II
I I
I I
I I
I I
I I
I I
I
I
I II
I
I
Filament I Ii
I II
I
I
I
I Ii
Electron Beam
u I
Focus Cylinder
Flight Tube
(-240v)
EI EP IEP OP G3
(-6vdc) (GD) (GD) (+3600v)
FIGURE 1. Schematic diagram ofhigh pressure ion source developed foratime-of-flight mass spectrometer.
ment are maintained at a negative potential of which increases the number of ions reaching the
-240 V. detector on any given pulse cycle.
The filament is mounted in such a way that the The ion source of the mass spectrometer is
electron beam path coincides with the ion axis. housedinside a T-shapedstainlesstubewhich was
This arrangement differs from that used in most fabricated for this purpose and which is con-
conventional ion sources, in which, the electron nected directly to the drift tube of the mass spec-
beam typically enters the side ofthe source cham- trometer. This housing incorporates a high speed
ber at an angle of 900 to the direction of ion ex- pumping port, to which a 4-in. oil diffusion pump-
traction. With the latter configuration, ions ing system is directly attached via a flexible 10-
formed in the region just above the ion exit aper- cm stainless steel bellows. For the source exit
ture are more likely to be extracted and detected. aperture currently used, this is adequate to main-
At lowersource pressure this conventional config- tain a pressure differential of about 1000:1 be-
uration functions well, since ions are formed uni- tween the source interior and the surrounding re-
formly along the electron path. However, as the gion. This differential pumping reduces collisions
source pressure is increased, the penetrating between ions and neutrals outside the source re-
power of the electron beam decreases rapidly. gion to a significant extent. The drift tube region
Thus, only a few ions are formed in the region ispumped bythestandardpumpingsystem which
near the exit aperture. With our new source de- consists of a 4-in. mercury diffusion pump, a
sign, all primary ions are formed along the elec- Bendix combined liquid nitrogen trap and Freon-
tron beam path (which is coaxial with the ion ex- cooled baffle, and a Welch forepump. The pressure
traction path) and are therefore produced in the is monitored by a Philips-type cold-cathode ioni-
region directly adjacent to the ion exit aperture. zation gauge (Veeco) which measures pressures
At higher pressures in the source, these primary as low as 2 x 106 mm Hg.
ions then react with other molecules in the same The sample inlet system is an all-metal dual
region to form the secondary product ions. As reservoir system, containing two 1-liter stainless
noted earlier, a repeller field is applied to drive steelexpansion tanks. Eachreservoiris connected
ions thus formed through the ion exit aperture, to the ion source via a variable gas leak, in one
November 1980 49
case, a Granville-Phillips Series 213 motor-driven ofthe computer. The digitalsignals arethen proc-
variable leak, and in the other, a Granville-Phil- essed by the computer using previously designed
lips Series 203 manually adjusted leak. Additions software developed in our laboratory.
to the sample inlet system were constructed re- Negative ion mass spectra were recorded with
cently to permit introduction of multi-component the TOF mass spectrometer, normally under the
reagent mixtures and gas samples, and to allow following conditions: accelerating voltage, 3.6 kV;
coupling of a gas chromatograph (GC) to the primary electron beam voltage, 240 eV; ion source
mass spectrometer. These additional inlet sec- envelope pressure, approximately 5 x 10-6 mm
tions are also controlled by Granville-Phillips Se- Hg; source temperature, 70-120°C; and the emis-
ries 203 manually adjusted leaks, and permit in- sion current (which is the current measured at
troduction of condensed phase samples from an the electron entrance plate) was adjusted to ob-
inlet tube. The latter is a U-shaped 1/4-in. ID tain maximum ion intensity. Usually, the latter
glass tube with one end sealed by a septum, and was varied from several tenths milliampere to
the other end connected to a Whitey SS-2RS4 about 10 mA. The source pressure was monitored
metric valve, which controls the molecular flow of by an MKS Baratron Type 77-H-1 pressure sensor
the sample vapor. This tube was installednearthe head. The sample pressure was maintained at a
source-housing flange in order to reduce con- constantvalue, usually between 0.5 and 10 .m and
densation in the inlet to a minimum. A MKS Ba- the total pressure was maintained at 100,m
ratron pressure sensor head (type 77-H-1) is at- (with the reagent gas added).
tached to the source housing to measure the ion Fluoroform used in these studies was obtained
source pressure directly. Alltubingusedfor inter- from Matheson Gas Products, East Rutherford,
connections in this system is 1.27 cm diameter N.J., and was specified to be approximately 99.9%
stainless steel tubing. Swagelok tube fittings are pure. NF3 was obtained from Air Products and
used for all couplings. A variety of values are also Chemicals Inc., Allentown, Pa., with a reported
used in the system. The sample inlet system and purityof99.92% minimum. Allorganiccompounds
the pressure reference side ofthe MKS sensor are studied were obtained from commercial suppliers
pumped by means of a CVC type PMCS-2B diffu- and were used without further purification.
sion pump, backed by a Welch 1400B forepump.
A Hewlett-Packard 2116C computer with 16-bit Results and Discussion
word length and 24K core memory was used for
data acquisition. Since the analog electrometer Formation ofthe Primary F- Ion
used forthe recordingofnegative ions isnot elec- Reactant with Fluoroform as the
trically isolated, the signals from the multiplier Reagent Gas
cannot be directly input into the computer. In-
stead, the output signal of the amplifier is further
Spectrum of Pure CHF3 Under NICI Condi-
amplified by a Keithley Instrument Type 109
pulse amplifier with 50 ohms input resistance and tions. In principle, the F- ion can be produced
20 DB gain. The amplified signal is then trans- from several gaseous fluoride compounds by elec-
mitted to a scan converter (Pacific Instrument), tron impact. The appearance potential of F- from
fluoroform (CHF3) is about 9.3 eV (11). Thus ini-
which is built into the oscilloscope. The computer
directly accepts the signal from the scan con- tially the latter molecule was selected as the re-
actant source molecule. The NICI mass spectrum
verter. This procedure introduces some noise, and
of CHF3 at a pressure of 0.1 torr and a temper-
reduces detection sensitivity considerably, but
ature of 100°C is shown in Table 2. It can be seen
permits direct computer acquisition of the data.
that electron impact on CHF3 at relatively high
Four channels are used to establish communica-
source pressure yields very small amounts of F-,
tion between the computer and the TOF mass
and large abundances of various cluster ions. Ap-
spectrometer. The first channel transmits a signal
parently this is due to the rapid association reac-
from the Y axis of the oscilloscope to the com-
tion of the F- ion with one or more HF molecules,
puter for the ion intensity. The second channel
to form cluster ions of the type, [(HF)nF]-,
transmits a signal from the X axis of the os-
cilloscope tothe computer forarampvoltage. The where n = 1-3 (14). There are two possible
third channel transmits the junction voltage of a sources for the HF molecules. One possibility is
the proton transfer reaction between F- and the
thermocouple used for temperature measure-
fluoroform molecule, in which CF3- is also formed
ment. The fourth channel carries a relay message
[Eq. (1)].
to the computer. These analog signals are con-
verted into digital signals by the A/D converter F- + CHF3->CF3- + HF (1)
50 Environmental Health Perspectives
Table 2. NICImassspectrumoffluoroformobtainedat A trace quantity of n-butanol (about 0.5,Lm)
100°C and0.1iLm ion source pressure. was injected into the ion source which also con-
Relative tained fluoroform at a pressure of 0.1 ,um. Molecu-
m/e Ion intensity,% lar, or quasi-molecular ions, such as (M - 1)- and
(M + F)-, were not observed in the NICI spec-
19 F-
39 (HF)F- 19.2 trum under these conditions. The failure to ob-
59 (HF)2F- 100.0 serve such ions possibly indicates that the reac-
69 CF3- 6.9 tion rateof F-withn-butanolismuch slowerthan
79 (HF)3F- 23.8 that of F- with neutrals such as HF.
85 CF30- 24.7
89 (CHF3)F-/(CF3)-(HF) 8.0
105 (CF30) (HF)- 8.3 Formation ofPrimary F- Ion Reactant
109 (CHF3) (HF)- 11.5 with NF3 as the Reagent Gas
128 (CF2) (CF30)- 9.5
Spectrum of Pure NF3 Under NICI Condi-
135 C2F50- 10.3
tions. Owing to the very low intensity of the
primary F- ion observed in the fluoroform NICI
The other possibility is a reaction of F radicals,
spectrum, a second reagent gas, NF3, was tested
generated in the initial electron impact event,
as a source of F-. The NICI spectrum of NF3 at a
with hydrogen-containing impurities (RH) resi-
total pressure of 0.1 ,um shows that the m/e 19, F-
dent within the ion source [Eqs. (2) and (3)].
is the major peak, with only a trace quantity of
CHF3 +e F-+2F+CH (2) m/e 39, HF2-. Again, the HF molecule with which
F- clusters here is probably formed from hydro-
F + RH-- HF + R (3)
gen-containing impurities in the source, by a
The enthalpy of reaction (1) is on the order of mechanism similar to that described previously.
-56.5kcal/mole, andthis process istherefore sub- NICI Spectrum of n-Butanol Obtained with
stantially exothermic. For many RH reactants, NF3 Reagent Gas. Table 3 shows the NICI spec-
reaction (3) would also be highly exothermic. trum of n-butanol obtained with NF3 reagent gas
Trace quantities of 02- or 0- ions inside the at a total pressure of 100,um andat a temperature
source could react with fluoroform molecules to of 70°C. About 0.5 ,um of n-butanol was injected
yield the observed m/e 85 peak corresponding to into the source with the NF3 for this experiment.
the (M - H + O) ion in the NICI spectrum The ions at m/e 19, 39, and 59 are presumably
shown in Table 2. The m/e 89 ion could be formed formed as described earlier. The m/e 93 likely
either by clustering between fluoroform and F-, originates from the directattachmentof F-to the
or by clustering between HF and CF3-. As shown n-butanol sample molecule, and the m/e 113 is
in Table 2, both the m/e 105 and the m/e 109 ions then formed byclusteringofan HF molecule with
can be explained on the basis of clustering of HF the (M + F)- ion. Apparently, F- undergoes com-
with the m/e 85 and 89 ions. There are also rela- peting reactions with the n-butanol and HF mole-
tively small amounts of m/e 128 and m/e 135 in cules. Several additional experiments were there-
the NICI spectrum. Itis tentatively assumed that fore conducted in which various experimental
the former ion is C3F40-, while the latter ion is parameters were varied, in an attempt to maxi-
apparently C2F50-, which probably arises from mize the intensity of the (M + F)- product.
HC2F5 or other such impurities in the fluoroform. Effects of Varying Experimental Conditions
Temperature Effects on the NICISpectrum of on the NICI Spectrum of n-Butanol Obtained
CHF, The relative abundances of ions observed with NF3 Reagent. Variation of the Total Pres-
in thefluoroform NICI massspectrumwere found sure of NF3. At an electron energy of 240 eV,
to be temperature-dependent. The variation in and a source temperature of about 70°C, the rela-
the relative intensities was studied over a tem-
perature range of 70 to 23000, at a total source
pressure of 0.1 ,um. It was observed that the rela- Table 3. NICI spectrum ofn-butanol (0.5gm) obtained
tive intensity of m/e 39 increases with increasing withNF3reagentgasatatotalpressureof100um
source temperature, while the intensities of m/e Relative
59 and 79 decrease, as dothose of the m/e 105 and mle Ions intensity,%
109 ions. It seems apparent that at higher source 19 F- 47.7
temperatures the formation of the larger cluster 39 (HF)F- 100.0
ions in fluoroform is less favored. This is consis- 59 (HF)2F- 64.5
tent with the increased relative intensity of the 93 (M+F)- 10.8
113 (HF) (M+F)- 3.2
m/e 69 ions as the temperature is increased.
November 1980 51
Table 4. Ratioofrelativeintensitiesofmle39to93inthe is that the production of F radicals is enhanced
NICI spectrum ofNF3-n-butanol atvarious pressures of significantly at higher electron energies. The F
NF3.
radicals thus formed react, in turn,with n-butanol
Relativeintensity,% to form more HF molecules. Thus, F- collisions
PNF3, Ratio with HF molecules are more probable than colli-
Jim I39 I93 139/193
sions with n-butanol molecules at the higher elec-
30 8.4 7.4 1.1 tron energy.
40 25.0 19.9 1.3 VARIATION OF THE REPELLER FIELD. By ap-
60 31.2 8.3 3.8 plying a small repeller field to the electron en-
80 59.8 10.2 5.9
trance plate of the ion source, (-6V, dc), the resi-
100 100.0 10.8 9.3
120 31.3 2.8 11.2 dence time of ions inside the source can be
reduced. However, the distribution of reaction
products was not observed to change significantly
tive intensities of m/e 39, (HF)F-, and m/e 93, as the repeller potential was varied over this
(C4H9OH)F-, in the NICI spectrum of an n-buta range.
nol-NF3 mixture were observed at various total VARIATION OF SOURCE TEMPERATURE. Vary-
ion source pressures overthe range from 40to 120 ing the ion source temperature over the range
,im. The observed intensities and the ratio of 100-190°C while other parameters are kept con-
these are shown in Table 4. The n-butanol sample stant, resulted in little change in the ratio of rela-
in these experiments was maintained at a con- tive intensities of m/e 93 to 39. Apparently, the
stant pressure of 0.5 ,um. It can be seen from competition between (HF)F- and (M + F)- pro-
Table 4 that the ratio of the relative intensity of duction is rather insensitive to temperature.
m/e 39 tothat of m/e 93 increases as the pressure However, the relative intensities of both the m/e
of NF3 increases. This indicates that at higher 39 and 93 ions, with respect to m/e 19, decrease
pressures the preferred reaction is the association markedly as the temperature increases.
reaction of F- with HF. The m/e 93 ion is formed VARIATION OF SAMPLE PRESSURE. A series of
via a second-order reaction, in which F- ions are NF3-NICI spectra of n-butanol were obtained at
first produced from NF3, and then cluster with n- various n-butanol pressures ranging from 1 to 40
butanol molecules. The (HF)F- product ions, on ,um at a total pressure of 80 ,um and with all other
the other hand, are formed by a reaction which is parameters held constant. Listed in Table 5 are
at least third-order. Probably, the F radical, pro- the relative intensities of the major ions ob-
duced by electron impact on NF3, abstracts a hy- served, and the ratios of relative intensities of
drogen atom from an n-butanol molecule or other two successive cluster ions. At higher n-butanol
hydrogen-containing impurities to form an HF pressures, large clusters are observed, m/e 79 and
molecule, which then clusters with F-. Thus, the 99, which result from successive HF additions. At
observed intensity of the (HF)F- ion varies as butanol pressures above 4 Am, a peak is observed
the third power of the pressure. At higher pres- at m/e 167. This probably corresponds to an M(M
sures, the reaction forming (HF)F- is thus more + F)- ion, which is formed by clustering of an-
favorable than that forming (M + F)-. Thus, in other butanol molecule with the (M + F)- ion.
order to observe a distinct quasimolecular anion The I59/1I39 ratio is seen to increase sharply as the
in the F- NICI mass spectrum, such as m/e 93 in sample pressure increases. Moreover, the ratios,
this example, the total source pressure has to be I79/I59, I99/I79, and Il13/I93 also increase proportion-
kept relatively low. However, the pressure cannot ately with pressure. This indicates that n-butanol
be lower than 30 ,um, or this product is not ob- plays an important role in the cluster ion forma-
served, owing to the characteristics of the ion tion. It is probable that n-butanol is the most
source employed in the present studies. significant source of hydrogen atoms forthe reac-
VARIATION OF THE ELECTRON ENERGY. The tions yielding HF molecules. With increasing n-
electron energy is another parameter of consid- butanol pressure, the yield of HF molecules there-
erable importance in NICI experiments. When fore increases. These HF molecules then react
the electron energy was lowered to 40 eV, while further with F- to produce cluster ions of various
all other parameters were kept constant for the orders. The ratio, 1167/I93, is also observed to in-
present gaseous system, the reaction leading to crease gradually as the sample pressure increases.
formation ofthe (M + F)- ion was foundto be fa- This is because at higher concentrations of
vored. The ratio of m/e 39 to m/e 93 was found to sample molecules, the formation of the (2M + F)-
decrease abruptly as the electron energy was re- ion is enhanced. Finally, as seen in Table 5, at
duced from 240 V to 40 V. A possible explanation sample pressures above 4,um, the cluster ion, m/e
52 Environmental Health Perspectives
59, formed by association of two HF molecules Negative Ion Chemical Ionization Mass
with F- is preferred to other cluster ions. Spectra from Reactions ofF- With
ADDITION OF OTHER GASES TO THE NF3 RE- Various Organic Compounds
AGENT GAS. In order to reduce the population of
highly energetic electrons in the ion source, cer- As described in the foregoing sections, impact
tain gases can be added to the reagent gas which of electrons on gaseous NF3 produces alarge pop-
act as electron scavengers. Removal of these elec- ulation of F- ions, which can be used as reactant
trons is desirable because the reaction of high en- ions to generate negative chemical ionization
ergy electrons with NF3 is considered to be the mass spectra. Since F- has a large gas phase pro-
major source of F radicals. In the present study, ton affinity, reactions of this ion with many mole-
the use of argon as ahigh-energy electron moder- cules to produce (M - 1)- products by proton
ator was investigated. Argon was mixed with the transfer are expected to be common. Further-
NF3 gas in different proportions, while maintain- more, F- is a strongnucleophile, and so bimolecu-
ing a total ion source pressure of 80 ,um. The n- lar nucleophilic substitution reactions with this
butanol sample pressure was maintained at about ion are expectedtobeobservedinsomecases. The
0.5 Am, while other parameters were kept con- organic compounds, for which F- NICI mass
stant, as before. It was observed that the ratio of spectra were examined in this study, were se-
relative intensities of m/e 39 to 93 obtained with lected as representative of avariety of functional
various pressures of Ar added to the source gas group classes. The detailed experimental results
mixture did not decrease, as was expected. obtained for each functional group class are dis-
In both solution and gas phase reaction studies, cussed in the following sections. The experimen-
oxygen has been employed as an inhibitor, owing tal conditions used in obtaining these NICI
to its ability to scavenge atoms or radicals, thus spectra were selected on the basis of the results
formingspecies which are incapable ofparticipat- obtainedinthepreviouslydescribedstudies of the
ing in the chain-propagation steps. It was consid- effects of pressure, temperature and other experi-
ered feasible, therefore, that the addition of oxy- mental parameters.
gen to the NF3-butanol gas mixture in the ion Carboxylic Acids. The F- NICI mass spectra
source might reduce the population of F radicals. of formic, acetic, isobutyric and mercaptoacetic
Small quantities of oxygen gas (pressure ranging acids are shown in Table 6. It is seen from these
from 2 to 6,um were therefore added to the datathatthe most intense ion in the spectraof all
source, in aseries ofexperiments, in which the to- thesecarboxylic acids isthe (M - 1)-ion. This ion
talpressure was maintained at 50,um, the n-buta- isformed by the abstraction of aproton, probably
nol pressure was kept constant at 2,um, and other from the COOH group. In mercaptoacetic acid,
parameters were notvaried. The ratio of the rela- both the SH and the COOH groups contain active
tive intensityofm/e93tothatofm/e39 observed hydrogen, but more likely, abstraction occurs
in the NICI spectrum was observed to increase by preferentially from the COOH group.
a factor of six, as the pressure of oxygen in- F- attachment to these carboxylic acids to form
creased from 0 to 6 ,tm. Thus, oxygen clearly ap- (M + F)- ions was also observed, as indicated by
pears to inhibitthe formation ofthe fluorine radi- the spectra shown in Table 6, but the relative in-
cal. However, the overall ion intensities decrease tensities of these ions are much smaller than
as 02 pressure increases, and in addition, the life- those of the (M - 1)- products. At higher ion
time of the tungsten filament is shortened, both source pressures, the NICI spectra of formic acid
of which are undesirable results. and acetic acid show clusters of these molecules
Table 5. RelativeintensitiesofmajorionsandratiosofrelativeintensitiesofsuccessiveclusterionsintheNF3-NICI
spectraofn-butanolatvariousn-butanolpressures.a
Relativeintensity,%
Pn-butanol, I59 I79 199 I113 I167
,um I19 I39 I59 I79 I93 I99 I113 I167 I39 I59 I79 I93 I93
1 8.2 19.5 7.3 1.5 2.0 0.7 0.37 0.21 0.35
4 18.3 86.1 80.0 19.8 12.7 5.6 1.3 0.93 0.25 0.44 0.1
8 16.4 28.9 100.0 28.7 4.4 11.7 7.3 - 3.46 0.29 0.41 1.66
12 14.8 13.8 100.0 19.3 5.0 14.3 8.0 5.0 7.25 0.19 0.74 1.6 1.0
16 9.8 9.8 100.0 21.9 4.1 15.3 10.5 5.5 10.2 0.22 0.7 2.56 1.34
40 4.2 1.1 100.0 44.6 5.1 7.4 13.0 8.2 90.0 0.45 2.55 1.61
aThetotalsource pressure wasmaintained at80,um.
November 1980 53
Table 6. F-NICImassspectraofcarboxylicacids (RCOOH).
Relativeintensity,%V Otherions
Relative
Compounds (M) Mol.wt. mle19 (M-1)- (M+F)- (2M - 1)- mle intensity Identity
Formicacid 46 21.2 100 2.4 38.1 42 5.5 CNO-
(HCOOH) 111 15.0 (HF) (2M-1)-
Aceticacid, 60 68.7 100 8.0 9.2 42 5.9 CNO-
CH3COOH
Isobutyricacid, 88 94.6 100 14.1 57 46.3 CH3COCH2
(CH3)2CHCOOH
Mercaptoacetic 92 73.4 100 2.7 59 35.2 CH3COO-
acid
HSCH2COOH
aIons lessthan 5% relativeintensity are omittedexceptthe quasi-molecular ions.
with the (M - 1)- ions, that is, (2M - 1)- ions. the esters studied here, with the exception of
Clusterions formed by association ofproduct ions ethyl formate. Thision is expectedtobe primarily
with HF molecules, were also observed in the for- formed by abstraction ofarelatively acidichydro-
mic acid NICI mass spectrum. A very weak in- gen from the molecule. In isopropyl acetate and
tensity ion at m/e 42 was observed in the spectra ethyl acetate, an alpha hydrogen is available for
of acetic acid and formic acid. Possibly, this ion is abstraction by the F- ion, but in methyl methac-
CNO-, as suggested by Alpin et al. (15), or rylate, an allylic hydrogen must be abstracted. No
(M - 1)- ion is observed in ethyl formate, since
CH2-CH2
this molecule contains no active hydrogen atoms.
Since alkoxide ion, R'O-, is more nucleophilic
than F- (16), the attackof F- atthecarbonyl-car-
as postulated by Field (10). The mechanism of
bon center, to release the alkoxide ion, does not
formation ofthis ion is unclear, but itseems more
occur with this group of ester compounds. This is
likely that CNO- would be formed than
demonstrated by the NICI spectrum of ethyl ace-
CH-IC 2 tate, since no m/e 45 peak (C2H50-) is observed.
Ions atm/e 26, 42, and46, havingveryweak in-
tensities, also appear in the spectra of these com-
because neither formic acid nor acetic acid con- pounds. In addition, a low intensity ion at m/e 41
tains an acetylenic group. Other observed ions appears in the spectrum of each ester except that
havingm/e values which donotcorrespond to any ofethyl formate. This ion probablycorresponds to
expected fragments for the compounds under- the ketene anion, -HC = C = 0 ± HC C-0-
going chemical ionization are quite possibly (15),which is rathercommonly observed from ox-
formedfrom impurities contained in the samples. ygen-containing compounds. The ion at m/e 61 in
Carboxylic Acid Esters. The F- NICI mass the spectra of ethyl acetate and isopropyl acetate
spectra of isopropyl acetate, ethyl acetate, ethyl is quite puzzling. It is possible that this ion is
formate, and methyl methacrylate are shown in -CH2COF. Two possible mechanisms can be con-
Table 7. Both isopropyl acetate and ethyl acetate ceived for its formation. One mechanism would
exhibit an intense m/e 59 peak in their F- NICI involve nucleophilic attack of F- on the carbonyl
spectra, while ethyl formate exhibits an intense carbon, resulting in displacement of the alkoxide
m/e 45 peak, and methyl methacrylate is charac- ion, yielding an acyl fluoride compound
terized by a strong m/e 85 peak. These peaks cor- (CH3COF), followed by abstraction of an alpha
respond to the carboxylate anions from each of hydrogen by F-. Another possible mechanism
these esters, CH3CO2-, HC02-, and CH2 = would involve addition of an HF molecule to the
C(CH3)CO2-, respectively, which are formed by ketene anion. As mentioned above, nucleophilic
an 0-alkyl cleavage, resulting from F- attack on displacement reactions involving attachment of
the alcohol alkyl group and subsequent dis- F- at the carbonyl carbon apparently do not occur
placement of carboxylate ions. This can be visual- with esters. Therefore, the first mechanism is not
ized as a bimolecular nucleophilic substitution re- probable. Other minor ions observed in the
action. spectra shown in Table 7 are again likely due to
A weak (M - 1)- ion is also observed in each of impurities.
54 Environmental Health Perspectives
Table7. F-NICImassspectraofcarboxylicacidesters(RCOOR')
Otherions
Relativeintensity,%a
Mol. Relative
Compounds (M) wt. m/e19 RCOO-1 (M - 1)- m/e intensity Identity
Ethylacetate, 88 21.4 100 (mle59) 3.4 39 23.6 (HF)F
CH3COOC2H5 61 10.2 -CH2COF
Isopropylacetate, 102 20.6 100 (m/e59) 3.5 39 18.4 (HF)F-
CH3COOCH(CH3)2 57 6.4 CH3COCH2
CH3COO- 61 8.4 CH2COF
79 5.2 (HF) (RCOO-)
Ethylformate, 74 45.8 100 (m/e45) 39 64.3 (HF)F-
HCOOC2H5 59 23.6 CH3COO or (HF)2F
CHOO- 65 28.9 (HF) (RCOO)-
Methylmethacrylate, 100 100 34.2 (m/e85) 0.4 26 6.8 CN-
CH2=C(CH3)COOCH3 39 16.4 (HF)F-
CH2C(CH3)COO 40 26.9 -CH2CN
45 8.3 HCOO-
59 19.6 CH3COO-or (HF)2F-
aIonsless than 5% relative intensity are omitted exceptthe quasi-molecular ions.
hR = H in the ethyl formate.
Ketones and Aldehydes. The F- NICI mass Similar to the ketones, the F- NICI mass
spectra of acetone, 2-hexanone, 4-heptanone, 2,5- spectra of aldehydes, including acetaldehyde,
hexadione, and cyclopentanone are shown in propionaldehyde and 2-ethylbutyraldehyde, are
Table 8. It is seen that the (M-1)- ion is the pre- all characterized by an intense (M - 1)- ion, as
dominant ion forallcompounds ofthis type which shown in Table 9. Undoubtedly, the reaction
were examined. The mechanism of formation mechanism in these cases also involves abstrac-
likely involves abstraction of the active alpha hy- tion of an active alpha hydrogen from the mole-
drogen in each of these compounds. In addition, cule. The (M + F)- ion is also aprominent species
(M + F)- ions, in varying relative abundances in the spectra of this group of compounds, and
(1.2-22%), are also observed in the spectra. These again, formation of these ions probably involves a
probably are formed by way of tetrahedral inter- tetrahedral intermediate. Analytically in-
mediates resulting from the attack of the F- ion significant ions, such as m/e 26, 39, 42, and 59, are
at the carbonyl carbon, this being a typical reac- all present in quite high concentratiQns.
tion ofketones and aldehydes (17). The molecular Aliphatic Alcohols. The F- NICI mass
anion M- is also detected in the spectra of these spectra of n-butanol, isobutanol, and tert-amyl al-
compounds, but in very weak intensity. This ion cohol are shown in Table 10. All of these com-
probably results from resonance electron capture pounds exhibit significant (M + F)- ions. In con-
by the molecule. An ion at m/e 26, apparently trast to the compounds previously discussed,
CN-, is also observed in each of the ketone however, aliphatic alcohols show no (M - 1)- ion
spectra. in their F- NICI spectra. This is likely attribut-
Table8. F-NICImassspectraofketones(RCOR').
Otherions
Relativeintensity,%a
Relative
Compounds (M) Mol.wt. m/e19 (M-1)- (M+F)- M- mle intensity Identity
Acetone,CH3COCH3 58 79.6 100 1.2 2.0 26 6.3 CN-
2-Hexanone, 100 34.0 100 13.0 4.6 26 11.6 CN-
CH3CO(CH2)3CH3 39 5.1 (HF)F-
4-Heptanone, 114 26.3 90.8 11.4 5.1 26 25.5 CN-
CH3CO(CH2)4CH3 39 13.4 (HF)F-
2,5-Hexadione, 114 42.9 100 22.0 5.8 26 6.1 CN-
CH3CO(CH2)2COCH3
Cyclopentanone 84 71.0 100 15.4 4.1 26 30.6 CN-
C5H80 39 23.1 HF(F-)
aIons less than 5% relative intensity are omitted exceptthequasi-molecular ions.
November1980 55
Table9. F-NICImassspectraofaldehydes (RCHO).
Otherions
Relativeintensity,%a
Relative
Compounds (M) Mol.wt. m/e19 (M-1)- (M+F)- m/e intensity Identity
Acetaldehyde,CH3CHO 44 15.7 100 7.0 26 42.1 CN-
39 15.5 (HF)F-
42 6.9 CNO-
Propionaldehyde,C2H5CHO 58 100 19.2 26 77.9 CN-
39 64.3 (HF)F-
42 14.7 CNO-
59 27.5 (HF)2F-
Ethylbutyraldehyde, 100 39.3 14.1 7.7 26 53.7 CN-
CH3CH(C2H5)CH2CHO 39 23.4 (HF)F-
59 100 (HF)2F-
79 36.8 (HF)3F-
aIons less than 5% relative intensity are omitted exceptthe quasi-molecular ions.
able to the fact that aliphatic alcohols are very apparently occurs via bimolecular nucleophilic
weak acids, and thus it is relatively difficult to re- substitution reactions. The observation of promi-
move a proton from the -OH group. nent Cl- product ions from these reactions sup-
As discussed earlier, the reactions of F- with ports the conclusion that the nucleophilicity of F-
the sample molecule are in competition with reac- is greater than that of Cl- in the gas phase (16).
tions of this ion with HF. It is evident from the Since F- is a stronger Lewis base than Cl-, it is
data in Table 10, that in the case of the alcohols, also possible for F- to abstract a chloronium ion,
formation of F-(HF) is more favorable than the Cl+, from the CC14 molecule to form a CCI3- ion,
formation ofthe (M + F)- ion. However, F- NICI althoughthis productis formedin relatively small
may be useful in distinguishing structural iso- amounts (1.4% relative intensity at m/e 117). The
mers of the alcohols. As can be seen by comparing characteristic chlorine isotope ratio leads to the
the spectra of n-butanol and isobutanol, the rela- observation of CCl3- ions at m/e 117, 119, 4nd 121,
tive intensities of the (M + F)- ion in the two in the intensity ratio, 3:3:1, which confirms the
spectra are markedly different. identity of this ion.
Alkyl Chlorides and Halobenzenes. The F- In the F- NICI mass spectrum of chloroform
NICI mass spectra of carbon tetrachloride, chloro- (CHCl3), an intense CCl3- ion is also observed
form, methylene chloride and 1-chlorobutane are (about 73.4% relative intensity at m/e 117). This
shown in Table 11. All ofthese compounds exhibit is consistent with the expectations of the "induc-
intense m/e 35 and 37 peaks, which correspond to tive" effect in this molecule. Chloroform contains
the two isotopes of Cl-. Formation of these ions three equivalent chlorine atoms located at the
Table10. F-NICImassspectraofaliphaticalcohols(ROH).
Otherions
Relativeintensity,%a
Relative
Compounds (M) Mol.wt. m/e19 (M+F)- m/e intensity Identity
n-Butanol 74 82.7 58.8 39 100 (HF)F-
CH3CH2CH2CH20H
59 11.0 (HF)2F-
113 6.9 (HF)(M+F)-
Isobutanol, 74 100 22.1 39 61.1 (HF)F-
CH3CHCH20H
CH3
tert-Amylalcohol, 88 51.5 17.5 26 12.0 CN-
CH3 39 100 (HF)F-
CH3-C-CH20H 59 20.9 (HF)2F-
CH3 93 19.6 (butanol+F)-
aIonslessthan 5%relative intensity areomittedexceptthe quasi-molecular ions.
56 Environmental Health Perspectives